How to enable USB debugging in Android?

Toggle on "USB Debugging" in the "Developer Options" area of Settings.

If you do not see "Developer Options", go into "About device" in Settings and tap on the "Build number" entry seven times, which will unlock "Developer Options".

As documented on Android Developers: https://developer.android.com/training/basics/firstapp/running-app.html

do this

1) go to settings

2) go to developers setting

3) enable developers setting by turning the switch on top of the page.

4) select Usb Debugging.

Note: if developer option is not available then Tap 7 times on the build version button to unlock developer options...